Vehicle manufacturers are implementing lighter, stronger materials, such as aluminum alloys, to meet emission reduction goals, meet fuel economy goals, reduce manufacturing costs, and reduce vehicle weight. At the same time increasingly demanding safety standards must be met while reducing vehicle weight. One approach to meet these competing interests and objectives is to hydroform high strength aluminum alloy tubular blanks into strong, lightweight parts such as roof rails and front rails of motor vehicles.
Aluminum tube types include seam-welded tube, extruded seamless tube, and extruded structural tube. Seam-welded tube and extruded seamless tube are expensive. Extruded structural tubes are lower in cost because they are formed in a continuous mill operation having a greater line and material utilization efficiency than extruded seamless tubes and seam-welded tubes.
Extruded structural tubes are formed by extruding an aluminum billet through an extrusion die at a high temperature and at high pressure. Discontinuous material flow across the section of the shape occurs when the flowing aluminum separates in the mandrel plate and re-converges in the cap section. A weld line, or joining line, is created where the flowing aluminum re-converges to form the extruded shape. Extruded structural tubes may have two or more weld lines that are an artifact of the porthole extrusion process.
Significantly, aluminum seam-welded, seamless tubes and structural extrusion tubes are less formable than mild steel tubes. As a result, it is not feasible to form most parts from aluminum tubes with state of the art conventional hydroforming operations. In such operations, a hydroforming fluid, such as water, is supplied to expand and shape the tube to conform to a die cavity. Substantial expansion of the tube (circumferential length of line expansion of more than 3%) is necessary to eliminate any tube buckles and produce complexly shaped parts.
In an effort to address this problem, a method of pressure sequence hydroforming is now proposed which is particularly adapted to use with aluminum materials. More specifically, this document relates to a hydroforming method that produces a part with a flat nose corner radius which functions to increase the formability ratio or R/t (tightness) of a hydroformed corner while simultaneously preventing splitting. The formability ratio or R/t is the ratio of the bend radius (R) to the workpiece/extruded aluminum tube wall thickness (t). This value defines the sharpest forming radius allowable prior to material failure. Advantageously, the resulting increase in the formability ratio allows one to effectively and efficiently use aluminum tubes, including extruded seamless and structural tubes, to produce vehicle parts such as rails by hydroforming.